1. Field of the Invention
The present invention relates to an electroluminescent device (EL device).
2. Background Art
In electroluminescent devices (EL devices), which use electroluminescence, attention is now focused on their use as light-emitting devices in various types of displays, and so forth. EL devices are self-light-emitting devices of injection luminescence type, which use luminescence that occurs at the instance electrons and holes arriving at a luminescent layer recombine with each other. The basic structure of EL devices is that a luminescent layer containing a luminescent material is situated between a cathode and an anode. EL devices are classified into inorganic ones using inorganic compounds as the luminescent material and organic ones using organic compounds as the luminescent material.
Recently, electroluminescent devices using, as the luminescent material, quantum dots have also been proposed (e.g., Patent Documents 1 to 3, and Non-Patent Document 1). Quantum dots are nanometer-sized fine particles of a semiconductor (semiconductor nanocrystals). Owing to their quantum confinement effect (quantum size effect) with which electrons and excitons are confined in nanometer-sized small crystals, quantum dots exhibit characteristic optical and electrical properties, and their utilization is expected in a wide variety of technical fields. A quantum dot emits light having a wavelength dependent on its particle diameter, so that it is possible to obtain lights different in wavelength by controlling the particle diameter. Further, since light emitted by a quantum dot is narrow in spectral width, it is excellent in color purity.
Although a layer containing quantum dots can be formed by a wet process which dispersion of quantum dots is applied, or a dry process which a material for quantum dots is deposited to form a film by such a technique as vapor deposition or sputtering, there is a tendency to adopt a wet process from the viewpoint of simplicity of apparatus and process, smoothness of the layer formed, and so forth.
However, using a wet process to form a quantum-dots-containing layer is disadvantageous in that quantum dots easily coagulate in their dispersion. For the purpose of controlling the dispersibility of quantum dots in a liquid and also the particle diameter of quantum dots in their production, the surfaces of quantum dots are protected by a protective material. Typical examples of protective materials effective in controlling the particle diameter of quantum dots in their production and in improving the dispersibility of quantum dots in a liquid include trioctylphosphine oxide (TOPO: [CH3(CH2)7]3PO).
Organic EL devices have a shortcoming that their life is short whether organic compounds used as the luminescent material are low or high in molecular weight. The reason for this is as follows: an organic compound used as the luminescent material deteriorates and decomposes during heat treatment in the production of a device, or due to heat a driven device generates, or because of chemical changes caused by transport of charges, formation of excitons, or emission of light, resulting in changes in emission characteristics including chromaticity, mobility of electrons, that of holes, and exciton production efficiency. In order to suppress such irreversible changes in emission characteristics that are caused by the deterioration and decomposition of an organic compound, many studies have been made until now.
For example, Patent Document 4 describes an organic electroluminescent device comprising organic compound layers, in which all the organic compound layers have glass transition temperatures of 75° C. or more, and an organic compound layer that is in contact with an organic blue-luminescent layer has a glass transition temperature of 105° C. or more. An object of the technique described in this patent document is to solve the following problems in the prior art: since the glass transition temperatures of blue-light-emitting organic compounds contained in organic EL devices are low, the devices are poor in thermal stability, undergo change in luminescent color, and deteriorate in luminous efficiency. Patent Document 4 describes that it is possible to solve these problems by using organic compound layers having glass transition temperatures of 75° C. or more to produce an organic EL device.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-38634    Patent Document 2: Published Japanese Translation of PCT International Publication for Patent Application No. 2005-502176    Patent Document 3: Published Japanese Translation of PCT International Publication for Patent Application No. 2006-520077    Patent Document 4: Japanese Laid-Open Patent Publication No. 1998-110163    Non-Patent Document 1: Seth Coe et al., Nature 420, 800-803 (2002)
On the other hand, quantum dots, which are made from inorganic compounds, hardly deteriorate even under high-temperature conditions, e.g., at a temperature of 75° C., and thus retain their emission characteristics such as luminescent color and luminous efficiency. However, we found the following: organic compounds, such as a material for protecting quantum dots and a binder component of a quantum-dots-containing luminescent layer, soften in the production or high-temperature storage of a device, or due to heat a driven device generates; this increases the mobility of the quantum dots in the luminescent layer to accelerate coagulation of the quantum dots.
Specifically, when dispersion of quantum dots is used to form a quantum-dots-containing luminescent layer (wet process), a solvent remaining in the film formed by the dispersion is an impurity for the luminescent layer, so that it is desirable to remove the remaining solvent by fully drying the film formed. This is because, if the solvent is remaining in the luminescent layer, there is the possibility that the device may deteriorate in driving stability (half-life of luminance).
The solvent to be used for dispersing therein quantum dots is selected depending upon the solubility of materials for the quantum-dots-containing luminescent layer, a technique to be used to apply the quantum dot dispersion, and others. For example, cyclohexane, toluene, xylene, anisole, mesitylene, tetralin, or cyclohexyl benzene can be used as the solvent; the boiling points of these solvents are 81° C., 111° C., 139° C., 155° C., 165° C., 208° C., and 240° C., respectively. To remove these solvents fully from the luminescent layer by drying, it is desirable to heat the film of the quantum-dots-dispersed liquid to temperatures above their boiling points. Therefore, in the case where any of the above solvents is used, it is necessary to dry the film at a temperature of 81° C. or more, even if cyclohexane, which has the lowest boiling point among the above solvents, is used.
If the material for protecting the quantum dots has a glass transition temperature and a melting point of less than 81° C., it softens in the step of drying its film. This increases the mobility of the quantum dots, accelerating coagulation of the quantum dots.
A solvent having a boiling point lower than that of cyclohexane (less than 81° C.) can also be used as the solvent in which quantum dots are dispersed. However, if such a low-boiling-point solvent is used, it is difficult to ensure the coating stability of the quantum dot dispersion in the step of applying the dispersion (by ink-jet printing, for example), which makes it difficult to control the thickness, pattern, etc. of the luminescent layer.
Conventional quantum-dot-protecting materials represented by TOPO have low glass transition temperatures (Tg) and melting points, so that they soften when heated in the production process of an EL device as described above, or during storage of a device, or due to heat a driven device generates. This increases the mobility of the quantum dots, accelerating coagulation of the quantum dots.
A quantum dot emits light whose color is dependent on its size, as is described above. Therefore, if the quantum dots coagulate in the quantum-dots-containing luminescent layer and thus undergo change in crystalline structure, the luminescent color changes, and, moreover, quenching occurs. This is one of the major causes for deterioration of EL devices in emission characteristics. Furthermore, coagulation of the quantum dots causes phase separation in the luminescent layer, which leads to non-uniform light emission; it also changes the mobility of electrons and holes in the luminescent layer and decreases the efficiency of exciton production, which leads to decrease in luminous efficiency. Namely, coagulation of the quantum dots in the luminescent layer that occurs in the production process or storage of an EL device, or due to heat a driven device generates, hinders the prolongation of EL device life, when viewed from the stability of emission characteristic of the luminescent layer.